Reducing NOx Emissions Using Fuel Staging, Air Staging, and Selective Noncatalytic Reduction in Synergy

Abstract

Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) are techniques for abating nitrogen oxides (NOx ) NO + NO 2 ) from boilers and engines. Each of these techniques has a limited range of applicability, within which 50% to 70% NOx reduction is attained. Higher reductions are achieved by methods that use the aforesaid techniques in sequence, thus cumulating their reduction ability, but also collecting their respective limits. In this paper, we describe a new method that we call "combined staging" (CS). This method combines FS, AS, and SNCR in synergy rather then in sequence. In CS, the fuel is first staged for converting NOx precursors to hydrogen cyanide (HCN). Then, the air is staged for reducing HCN to N 2 . Further reduction is achievable by optional SNCR. In the followings the basics of FS, AS, SNCR, and their sequential applications are reviewed first. Then, the combined staging is introduced, its chemical details are elucidated via kinetic modeling, and options for its application are illustrated. Finally, assumptions and limits of the kinetic models are discussed. The present work reveals that CS can reduce over 40% NOx at lower temperatures and within shorter residence time than required by other techniques and methods. Thus, CS could reduce NOx effectively in devices where other techniques fails, e.g., in kraft recovery boilers, fluidized bed combustors, low-grade fuel combustors, small and domestic boilers, and fast engines.

Experimental Investigation on NOx Reduction by Primary Measures in Biomass Combustion: Straw, Peat, Sewage Sludge, Forest Residues and Wood Pellets

“…These measures include staged air and staged fuel combustion, modification to the fuel composition by fuel blending, co-combustion, improvements to the combustion chamber, flue gas recirculation, fuel pretreatment, etc. [4,[35][36][37][38][39][40][41][42][43][44]. e.g., the use of biomass/coal cofiring has become more interesting due to the increased environmental performance of energy production from solid fuels at a moderate cost.…”

“…However, the operation range and the overall economics of the plant should be considered before applying such costly methods [16,41,46,47]. Therefore, for small biomass combustion reactors, only primary reduction techniques are considered used in a feasible way.…”

“…Some researchers tried to use a combination of the methods. Such combined staging (CS) was developed by Zabetta et al [41] who used staged air, staged fuel and selective noncatalytic reduction (SNCR), and differs from other approaches since it pursues the reduction of NO x via HCN as the key intermediate. The application of CS method is relative for the small burner or very fast engines where the residence time is too short to reach the maximum reduction from the simple staging technique.…”

Reburning in Oxy-Fuel Combustion: A Parametric Study of the Combustion Chemistry

“…(1−θ) (1−θ)+θX red (4) Furthermore, the oxygen concentration of the oxidizer (C O2 ) affects the amount of NO x recycled to the furnace (θ), …”

Optimization of air staging in a 1 MW tangentially fired pulverized coal furnace

“…NO x reduction efficiency of air staging ranges from 20% to 50%, and it depends on a boiler's initial NO x level, fuel combustion equipment design, and fuel type [3,11].…”